When groundwater is deep, or the ground is hard to dig, boreholes are generally quicker and less expensive to construct than dug wells. Most boreholes are lined with either a 100 mm or 150 mm steel or plastic rising main, so the pump must be small enough in diameter to fit down it.
Ideally, bore hole diameters need to be kept small (large boreholes are very expensive to drill) the best way to obtain a larger output from a borehole pump is to increase its stroke. The speed of operation of borehole pumps is usually restricted to about 30 strokes a minute, although a few operate at up to 50 strokes per minute. Higher pumping speeds tend to buckle the long train of pump rods by not giving the pump rods and piston sufficient time to fall back on the down stroke under gravity. Obviously, with any depth of borehole, it is important to avoid compressing the pump rods, or they may buckle and jam against the sides of the rising main.
Most simple borehole pumps are reciprocating pumps. A reciprocating pump moves water in a non-continuous manner, so the water is constantly accelerated and decelerated by the movement of the piston. Very large forces can be created if long pipelines containing a large mass of water are directly connected to a reciprocating pump. This is because the pump piston tries to force the water in the pipeline to move rapidly from rest to speed, and then back to rest; and since water is incompressible it will try and follow the motion of the piston. Therefore reciprocating pumps generally need to be isolated from water in long pipelines, in order to cushion the water in the pipeline from the motion of the piston.
Three methods for isolating reciprocating pumps from pipelines are illustrated in FIG. 1, namely:                A. Open riser (also avoids need for pump rod seal) but only suitable where riser can be above pipeline discharge level;        B. Gravity pipeline from small tank near pump; and        C. Compensator tube or stuffing box; necessary when discharge is significantly higher than pump.        
When the pump outlet is set close to or above the pipeline discharge level, there is no great problem because the pipeline can be de-coupled from the pump by feeding into a small tank which can then gravity feed the pipeline steadily; see FIG. 1B. Alternatively, a riser open to the atmosphere in the pipeline near to or over the pump can achieve the same effect (as in FIG. 1A); because the pump rod can go down the riser it neatly avoids the need for a seal or stuffing box.
Where the pump delivers into a pipeline which discharges at a significantly higher level, it is generally not practical to have a riser open to the atmosphere at or near the pump, since it obviously would have to extend to a height above the level of the discharge. The solution generally applied in such cases where more than a few meters of suction or delivery line are connected to a reciprocating pump is to place an air chamber or other form of hydraulic shock absorber between the pump and the pipeline (FIG. 1C).
When water from the pump seeks to travel faster than the water in the pipeline it will by preference flow into the air chamber and compress the air inside it. When the piston slows so that the water in the pipeline is travelling faster than that from the pump, the extra water can flow out of the air chamber due to a slight drop in pressure in the pipeline and “fill the gap”. In other words, an air chamber serves to smooth the flow by absorbing “peaks” in a reciprocating output and then filling the “troughs” that follow the peaks.
An air chamber is preferably placed as close to the pump as possible but normally outside the borehole in order to allow maintenance of the air chamber and also to minimise the size of the equipment in the borehole.
Further, the friction from the passage of the water within a smaller riser pipe, particularly one with a large bore pump can create problems.
It would therefore be an advantage if the assembly were provided which could keep the water within the rising main moving, reducing the stop/start motion of the water previously associated with reciprocating pumps thereby reducing the chance of rod breakage, strain and wear on the pump barrel and buckets, and minimize the chance that the pump casing can lift within the borehole or well.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.